Holy Shock is a unique instant cast spell. It can be used either as a "clutch" heal or a powerful "nuke" spell.
Holy Shock should be used on cooldown since it's a Holy Paladin's primary source of Holy Power.
As a spell from the Holy School, using it offensively with [Righteous Fury] can generate needed aggro when off-tanking. A quick jolt of holy energy with at least 160% aggro generation will grab attention.
This ability was significantly buffed in the 3.0 patch so that it can now be a reliable spell in any healing rotation.
In Warcraft III the paladin hero's Holy Light spell healed friendly living units and damaged enemy undead units. Considering this, some may view Holy Shock as a variation of WC3 Holy Light.
Patch 6.1.0 (24-Feb-2015): Holy Shock damage has increased by 50%.
Patch 6.0.2 (14-October-2014): Holy Shock now has twice the normal chance to critically strike (instead of having an additional 25% chance to critically strike).
Patch 5.4.0 (10-Sep-2013): Holy Shock's mana cost has been reduced by 50%.
Patch 5.0.5 (11 September 2012): Holy Shock’s base damage has been reduced, and its spellpower coefficient has been increased.
Patch 4.2.0 (2011-06-28): Holy Shock mana cost has been decreased to 7% of base mana, down from 8%.
Patch 4.0.6 (8-Feb-2011): Crusader Strike, Holy Shock, and Hammer of the Righteous no longer incorrectly grant Holy Power from a missed attack.
/Patch 3.0.2 (14-Oct-2008): Holy Shock (Holy) range changed to 20/40 yards when used on enemies/friends respectively, cooldown reduced to 6 seconds.
Patch 2.4.0 (25-Mar-2008): Casting lower ranks of these spells is now properly penalized like other healing and damage effects.; The healing, damage, and mana cost of this spell have all been increased.
Patch 1.10.2 (02-May-2006): Bug fix: Now guaranteed to crit when used offensively while Divine Favor is active.